1. Field of the Invention
The present invention relates generally to processes for generating electrical power by use of hot geothermal aqueous liquids and more particularly to processes for controlling the formation of scale in geothermal brine power plants and associated brine injection equipment.
2. Discussion of the Prior Art
Large subterranean reservoirs of naturally occurring steam and/or hot aqueous liquids (water or brine) have been found in many regions of the world. Such geothermal reservoirs are especially prevalent where the thermal gradient near the earth's surface is abnormally high, as in regions of volcanic, geyser or fumarole activity, as is commonly found along the rim of the Pacific Ocean.
In some regions, where relatively abundant and readily accessible, hot geothermal fluids have, for some time, been used for therapeutic treatment of bodily disorders, in industrial processes, for heating purposes and the like. Although effort in further developing geothermal resources for such purposes continues, substantial effort has recently been directed towards using geothermal fluids to generate electric power which is usually mush less site-restricted than is the more direct use of geothermal fluids for the above mentioned purposes. These interests in geothermal resources for power generation have been heightened by recent steep increases in petroleum and natural gas costs, as well as by the actual or threatened shortages of such fuels.
The general processes for using hot geothermal fluids to generate electric power are quite well known. For example, geothermal steam can, after treatment to remove particulate material and polluting gases, be used in the manner of boiler-generated steam to drive conventional steam turbine-generators. Naturally pressurized, high temperature (above about 400.degree. F.) geothermal water or brine is typically flashed to a reduced pressure to release steam which is used to drive steam turbine-generators. Lower temperature geothermal liquids are, in contrast, generally useful in binary fluid systems in which a low boiling point working fluid is vaporized by the hot geothermal liquid and the vapor is used to drive gas turbine-generators.
As can be appreciated, geothermal steam is preferred over geothermal liquids for the production of electric power because the steam can be used almost as extracted from the earth in generally conventional steam-turbine power plants. As a result, where abundantly available and favorably located, as at The Geysers in California, geothermal steam has been used for a number of years to generate substantial amounts of electric power at competitive costs. Unfortunately, however, abundant sources of geothermal steam are relatively scarce, and at current estimates are only about one-fifth as prevalent as good sources of geothermal aqueous liquids.
Because of the maturity of geothermal steam power generating processes and the scarcity of large geothermal steam sources, much of the current development effort in the geothermal field is directed towards developing commercially viable geothermal water/brine power generating facilities; particularly in such regions as the Imperial Valley in Southern California, where there is an abundance of geothermal brine.
General processes and techniques for using geothermal aqueous liquid to generate electric power are, as above-mentioned, known. Such processes and techniques are, in theory, relatively straight forward. However, in actual practice many serious problems are usually encountered in handling the geothermal aqueous liquids, particularly the brines. Geothermal aqueous liquids typically have wellhead temperatures of several hundred degrees Farenheit and pressures of several hundred p.s.i.g. and are typically heavily contaminated with dissolved materials. For example, in many regions, the geothermal aqueous liquids contain high levels of dissolved gases, such as hydrogen sulfide, carbon dioxide, and ammonia, as well as high levels of metals, such as, lead, iron, arsenic, and cadmium. In addition, many hot geothermal aqueous liquids are saturated with silica and many are also highly saline, in nature, being therefore termed brines.
Because of their high levels of contaminants and high wellhead temperatures, most geothermal aqueous liquids are not only corrosive to equipment and have scale forming characteristics, but the reduced-energy, geothermal effluent discharged from the power generating facility cannot be easily disposed of, particularly considering that flow rates in excess of one million pounds per hour are not uncommon. Effluent contaminants, such as lead and arsenic, preclude safe use of the discharged liquid for such otherwise potential uses as crop irrigation, and in most localities discharging of the effluent into rivers, lakes and other water supplies is prohibited. Ponding and evaporation of the discharged geothermal effluent is generally impractical because of the large volumes involved. Moreover, because of their typical heavy metal content, the evaporated residues are usually considered hazardous or toxic wastes and disposal is accordingly costly.
The most, and often the only, practical manner of disposing of the geothermal effluent is, therefore, by pumping it back into the ground through injection wells. Additional advantages of this method of disposal are that ground subsidence which might otherwise be caused by depletion of underground geothermal reservoirs is eliminated, and useful life of the underground reservoirs are usually increased.
Although reinjection often provides the only feasible method for disposing of geothermal effluent, serious problems, usually related to high silica content of the geothermal liquid, are nevertheless associated with such disposal. As mentioned, in many locations, the hot pressurized geothermal liquid, as extracted, is saturated with silica. When the geothermal liquid is flashed to extract steam for power production, the pressure of the liquid is reduced and the liquid becomes supersaturated with silica. As a result, silica rapidly precipitates from the liquid to form a hard scale on downstream piping and injection equipment, including the injection wells themselves. With many geothermal aqueous liquids, a silica scale formation rate of several inches per month is not unusual. As scaling of the piping, equipment and injection wells builds up, the geothermal liquid flow through the system becomes choked off and facility shutdown is then necessary for system reconditioning, which may include costly reboring of the injection wells. Because the silica scale is ordinarily very hard and tough, and clings tenanciously to equipment, the renovation process is difficult, time-consuming and costly, both in terms of actual renovation costs and in terms of nonproductive facility downtime.
Two general methods are typically used to minimize the silica scaling problems in geothermal liquid power producing facilities. One method is to treat or handle the geothermal liquid in such a manner as to keep the silica in solution through reinjection. The other method is to cause sufficient silica precipitation from the geothermal liquid, in a controlled manner and in specific facility stages from which the precipitated silica can be easily removed, to keep the silica level below saturation during the reinjection stage.
As can be appreciated, when the geothermal aqueous liquid is saturated with silica at wellhead temperatures and pressures, it is very difficult to keep the silica in solution when the liquid temperature and pressure is substantially reduced during the energy extraction process. The silica scale preventing method of controlled removal of sufficient silica so that the silica saturation level is not exceeded during the energy extraction process, although not without problems, may, therefore, be preferred in many instances where silica scaling would otherwise be a problem.
One of the greatest difficulties with silica removal processes is the removal of the right amount of silica at the right stage in the system. If an insufficient amount of silica is removed, silica scaling will not be prevented and if the silica is not precipitated where intended, the precipitate may carry over into other stages of the system and cause flow restriction problems. On the other hand, excessive removal of silica may overload the silica disposal stages and add to the silica waste disposal costs. Therefore, to assure a practical and relatively trouble-free system, the silica removal process must be carefully controlled.
With respect to the silica removal process, seeding of the geothermal aqueous liquid with a seed material, onto which the silica in solution crystallizes, appears to offer advantages of rapid, and hence location-controlled, silica removal. Such seeding processes typically pump some of the silica precipitate removed from one stage of the system into the flow of geothermal aqueous liquid at an upstream point, typically a flash-crystallizing stage which may be comprised of one or more flash-crystallization vessels. As the flashed geothermal liquid is contacted with the silica seed material in the flash crystallization stage, silica crystallizes from the liquid onto the seed material; the resulting precipitate is then removed, for example, in a downstream reactor-clarifier stage.
Problems have heretofore, however, been associated with disposing of the large flow of high pH steam condensate which results from using the steam extracted from the geothermal aqueous liquid. Typically the flow of condensate is about 10 percent of the flow of flashed geothermal liquid and may accordingly be as great as several hundred thousands pounds per hour. Although the steam extracted from the geothermal aqueous liquid by the flashing process is generally much less contaminated than the geothermal liquid, it usually has enough contaminants, notably boron and arsenic, which are carried over into the steam to cause the steam condensate to be unusable and, as in the case of geothermal liquid, the most practical disposal method for the condensate is reinjection. Therefore, the basic steam condensate is ordinarily recombined with the acidic, flashed geothermal liquid upstream of the injection stage.
Applicants have, however, discovered that because of the substantial differences in the chemical composition and also the pHs of the steam condensate and the flashed geothermal aqueous liquid, combining of the steam condensate with the flashed geothermal liquid upsets the chemical equilibrium in the liquid, thereby disrupting the silica crystallization process. Moreover, applicants have found that such recombination also causes the formation of fine particulate matter, for example, heavy metal sulfides, carbonates, and/or hydroxides, which remains in suspension and subsequently cloggs up media filters through which the combined geothermal liquid and steam condensate are passed before reinjection. Still further, equipment scaling has been discovered by applicants to occur in regions of condensate-flashed liquid recombination.
It is, therefore, an object of the present invention to provide a method for combining high pH steam condensate with an acidic, flashed, silica-rich geothermal aqueous liquid in a silica precipitating-type of system so as to prevent the formation of unwanted, suspended particulate matter.
Another object of the present invention is to provide a method for combining high pH steam condensate, within a silica removal stage, with flashed, silica-rich geothermal aqueous liquid, which includes adjusting the pH of the steam condensate so as to prevent the formation of undesirable, suspended particulate matter.
A further object of the present invention is to provide a method of combining, in a silica crystallization stage, a flow of high pH steam condensate with a flow of acidic, silica-rich, geothermal aqueous liquid, in which the pH of the steam condensate is adjusted so as to optimize the silica precipitation in the silica crystallization stage.
A still further object of the present invention is to provide a method for combining a flow of high pH steam condensate with a flow of hot, acidic geothermal aqueous liquid containing heavy metals in solution, which prevents the formation of suspended heavy metal compounds.
Still another object of the present invention is to provide a method for combining a flow of high pH steam condensate with a flow of hot, acidic geothermal aqueous liquid in which at least part of the steam condensate is used as a pump seal purge for pumps used in the system.
Additional objects, advantages and features of the invention will become apparent to those skilled in the art from the following description, when taken in conjunction with the accompanying drawing.